A ferrule compression fitting is commonly used as a connector and comprises one or more ferrules to be threaded over a tube or the like and a tightening device to cause the one or more ferrules to grip the outside of the tube or the like to provide a seal against leakage. GB 928,715 and U.S. Pat. No. 6,131,963 show examples of twin ferrule connectors. WO 2005/019716 discloses a tube fitting for a tube end, the tube fitting comprising a body with an interior bore to receive a tube end and a collar to be threadably coupled onto the body. The collar includes an integral tube gripping ring that co-operates with a camming surface on the body when the collar is threaded onto the body to force the ring to plastically deform into the tube end.
WO 2005/019717 discloses a tube fitting for a tube end, the tube fitting comprising a female threaded coupling member and a male threaded coupling member that are threaded together to assemble the fitting onto a tube end. Whilst being threaded together, a ferrule is detached from the female threaded coupling to form a separate ferrule fitting which is plastically deformed against the outer wall of the tube end and forms seals with the male coupling member and the tube end.
The following problems are associated with these designs and a variety of methods have been used in the past to overcome these issues.                The design relies on the use of controlled material specifications to ensure that the ferrules have enhanced mechanical properties over that of the tube.        The rear ferrule is treated by one of many processes to enhance the surface hardness. This in itself can cause issues as traditional hardening methods such as nitriding induces carbon into the material, which reduces the surfaces corrosion resistance.        Tube selection is critical within the design. The tubing used with these fittings often has to be purchased with special restrictions of some properties such as skin hardness.        Incorrect tube preparation can cause major problems within the assembly of the fitting and this is one aspect that the manufacturer has no control over.        Longitudinal scratches on the tube can cause leak paths with these fittings. Basic tube handling commonly causes these scratches. These minor surface defects are often manually removed prior to assembly, which adds to the assembly time.        Due to the requirement of enhanced materials it is accepted that compression fittings manufactured from austenitic stainless steel are not suitable for use within certain environments and do not comply with the NACE standards.        The method used to assemble a tube into a compression fitting relies on the end user adjusting the fitting until tube grip is achieved such as by using a threaded connection. This is detected by feel and thus every user will have their own opinion of when this is met. Once this point has been achieved the nut of the fitting is tightened by 1¼ turns and not to a specified torque or a dead stop. This is seen by a limitation within certain fields such as the automotive industry.        
These problems are addressed in GB2423561 in which a coupling is described having an inner body with an interior bore passing through at least a portion thereof to receive a tube or the like, the bore having one or more interior annular protuberances extending into the bore, and a collar to be provided around the inner body to apply a circumferential compressive force to deform the body. In use, when a tube or the like is received in the bore of the body and the collar applies a circumferential compressive force thereto, the body is deformed such that the one or more annular protuberances engage the tube or the like to seal with it.
It has been found that in when a torque is applied to the fitting and/or the tube, relative rotation may occur between the body and the tube. Deformation of the tube by the annular protuberances of the body may not be uniform about the circumference of the tube. During relative rotation, high and low points generated at the interface between the annular protuberances and the tube may become misaligned which can lead to a failure of the seal between the tube and the body.
It is therefore desirable to provide an improved coupling which addresses the above described problems and/or which offers improvements generally.